The present invention relates to corrugated pipe and its use in tubular strings conveying fluid through earth material, for example as part of a buried pipeline or casing in a well.
The invention was initially developed as a means to reduce thermally induced axial load in the production casing string of a well undergoing cyclic steam stimulation. The production casing strings in such wells are normally cemented in place and are therefore largely constrained from expanding or contracting axially during heating and cooling cycles. This constrained thermal strain is manifested as axial load which becomes more compressive during heating and more tensile during cooling. Depending on the thermo-mechanical material properties of the casing and the magnitude of temperature cycling, the axial stress may exceed the axial yield strength of the pipe in compression during heating and may exceed the axial yield strength in tension during cooling. Among other consequences, the high stresses place severe demands on the structural and sealing capacity of the tubular connections between casing joints and significantly reduce the ability of the pipe body to withstand collapse, bending and shear loads which may arise from various hydraulic and geomechanical factors. The incidence of leakage, fracture and access impairment xe2x80x98failuresxe2x80x99 is therefore relatively high in connection with the casing of thermal process wells.
Approaches taken by the industry to address this problem have typically included improving the strength and leakage resistance of the connections by utilizing more complex designs, for example substituting premium connections for the standard 8-round or buttress threadform connections, or increasing the grade of steel used. These approaches, while potentially providing significantly better seepage control and modest incremental structural performance, tend to increase cost and do not substantially reduce the risk of fracture or deformation induced failure.
Therefore there remains a need to address the primary confounding variable, namely the high axial stress induced by confined thermal expansion and contraction.
While thermal well design has been the primary motivator for the present invention, it is not to be limited to this application. The invention finds use in situations where there is interaction of loads between tubulars, surrounding earth material and contained or excluded pressure fluids, and where it would be desirable to increase axial or flexural compliance, decrease effective axial yield load and increase collapse resistance. One such situation involves buried pipelines. Here axial and flexural strain due to tubular-soil interaction must be absorbed without loss of pressure integrity. It would be desirable to provide tubulars of reduced axial and therefore flexural stiffness because these properties result in lower axial and bending loads than straight pipe for the same temperature variations and deformation magnitude.
The phrase xe2x80x9cstring of jointsxe2x80x9d as used herein is intended to encompass a plurality of joints of metal pipe, usually steel, connected end to end either by welding or threaded connections and to further encompass a sand exclusion liner if such a part of the string. The phrase xe2x80x9cthick-walled pipexe2x80x9d is intended to mean substantially rigid high pressure pipe useful as oil country tubulars, such as well casing and in high pressure pipelines, said pipe having a diameter to wall thickness ratio (xe2x80x9cD/txe2x80x9d) less than 100, preferably less than 50. The word xe2x80x9cformedxe2x80x9d is intended to mean that a cylindrical metal pipe wall has been plastically deformed by hydroforming, rolling or hydrofolding, preferably triaxial plane strain hydroforming.
The present invention applies a well known mechanical design concept, corrugations, to thick-walled metal pipe which is to be used in earth-restrained applications, such as in a string of joints used as casing in a well or as part of a pipeline. The corrugations are incorporated for the purpose of managing changes in axial load subsequent to installation.
More specifically, the invention involves forming thick-walled pipe to convert at least part of its cylindrical side wall into a sinusoidally corrugated configuration. The corrugations are formed so as to have a corrugation radius of curvature to thickness ratio (xe2x80x9cR/txe2x80x9d) less than 10, preferably less than 5. Preferably the corrugation webs have a maximum angle equal to or greater than 20xc2x0 with respect to the pipe axis. More preferably the corrugations have thinned webs and flattened peaks. Preferably, the pipe is hydroformed, without substantially changing its original length, to create the corrugations. By selecting the geometry defined by these limitations we have balanced axial compliance (i.e. reduced axial stiffness) with diametral limitations arising from the cost of increasing annular space consumed in a wellbore and material strain capacity.
Broadly stated then, in one embodiment the invention is concerned with a string of joints of thick-walled pipe extending through and being restrained by earth material, the string being subject to a change in axial load subsequent to installation, the side wall of at least one such joint having been formed into corrugations along at least part of its length, the corrugations having an R/t ratio less than 10. Preferably, one or more of the following conditions apply:
the string is used in a well and is subject to changes in axial load arising from thermal expansion or contraction (for example where the well is involved in cyclic steam stimulation) or from earth movement;
the string forms part of a buried pipeline;
the R/t ratio is less than 5;
a plurality of corrugated joints are distributed in spaced apart alignment along the string;
the corrugation webs have a maximum angle equal to or greater than 20xc2x0 relative to the pipe axis;
the wall thickness of the webs of the corrugations are thinner than the peaks;
the corrugations having been formed by hydroforming, more preferably while maintaining the length of the joint substantially constant;
the corrugations varying in wall thickness along their length, as a result of having been formed.
In another embodiment, the invention is concerned with a thick-walled steel pipe having threaded ends, the body of the pipe between the ends having been hydroformed to produce corrugations along at least part of its length, the corrugations having an R/t ratio less than 10. Any of the previously mentioned preferred conditions also may be incorporated.